1. Field of the Invention
The present invention relates to a secondary battery capable of improving the safety thereof and a safety device used for the same.
2. Description of the Prior Art
In general, secondary batteries are rechargeable batteries, which can be fabricated in a compact size or a large size. For instance, the secondary batteries include Ni-MH batteries, lithium batteries and lithium ion batteries. In addition, the secondary batteries are classified into cylindrical secondary batteries and square type secondary batteries according to the external appearances thereof.
Different from a pouch type secondary battery, the cylindrical secondary battery or the square type secondary battery includes a battery can serving as a positive electrode terminal or a negative electrode terminal and an electrode assembly accommodated in the battery can and including a positive electrode (cathode) coated with a positive electrode active material, a negative electrode (anode) coated with a negative electrode active material, and a separator interposed between the positive and negative electrodes.
Meanwhile, if a sharp tool, such as a nail or a drill tip, penetrates the positive and negative electrodes coated with active materials by passing through the battery can or if the battery can is pressed by means of a pressing tool, such as a nipper, an internal short circuit may occur between the positive and negative electrodes, so a great amount of current is applied between the positive and negative electrodes while generating heat. In an extreme case, the battery is subject to accidental ignition or explosion. For this reason, a safety device is provided in the battery in order to prevent the accidental ignition or explosion of the battery.
For instance, Japanese Patent Unexamined Publication No. 2000-48852 discloses a safety device for a secondary battery as shown in FIG. 1, wherein the safety device is provided at the outermost portion of an electrode assembly in the form of a jelly-roll, which is accommodated in a battery can of a cylindrical battery, by extending uncoated positive and negative electrodes, on which the active materials are not coated, by a predetermined length. The battery can is electrically insulated from the uncoated positive electrode or the uncoated negative electrode by means of a separator and the battery can may serve as a positive electrode terminal or a negative electrode terminal of the battery.
According to the above prior art, if the sharp tool penetrates the electrode assembly through the battery can, the short circuit occurs between the uncoated positive and negative electrodes having a relatively lower resistance, and also the short circuit occurs between the positive and negative electrodes having a relatively higher resistance, which are formed with active materials. At this time, short current may primarily flow between the uncoated positive and negative electrodes having a relatively lower resistance, so that an amount of current applied to the positive and negative electrodes having the active materials is reduced. Thus, the battery can be safely maintained even if the internal short circuit occurs in the battery, thereby preventing heat from being suddenly generated and the temperature from suddenly rising in the battery.
However, according to the above-mentioned conventional cylindrical battery, the short circuit may occur only when the sharp tool, such as a nail or a drill tip, penetrates the uncoated positive and negative electrodes after passing through the battery can, so that charged current in the battery can may not be rapidly discharged to the exterior while the sharp tool is penetrating into the battery can.
In addition, the uncoated positive and negative electrodes are aligned adjacent to the positive and negative electrodes coated with the active materials and the uncoated positive and negative electrodes have the thickness relatively smaller than that of the battery can. Thus, if the sharp tool introduced into the battery can any further moves toward the positive and negative electrodes coated with the active materials and penetrates the positive and negative electrodes, high-temperature heat may be suddenly generated in the battery can, so that the battery is subject to accidental ignition or explosion.